1. Field of the Invention
The invention relates to the art of suppressing interference present in a swath echo signal received by a wideband radar apparatus. More particularly, the invention relates to a device and method for facilitating this interference suppression utilizing a series of narrowband solutions.
2. Description of the Prior Art
Modern radar systems having ultrahigh resolution capability are useful in many applications such as target detection and terrain imaging. Such radars require the transmission and reception of radio frequency (RF) signals having a wide bandwidth commensurate with the desired resolution. This bandwidth requirement becomes extremely large at ultrahigh resolution. For example, a 500 megahertz (MHz) bandwidth is needed to support a resolution of one foot.
One radar typically used for these purposes is a synthetic aperture radar (SAR). A SAR utilizes a relatively small antenna aperture translated to have relative movement with respect to the target. At each of a series of sequential positions, a wide bandwidth signal is transmitted. Swath echo signals received in response to each transmission are placed in electronic storage, where it is important that both amplitude and phase information be maintained. After a sufficient number of signals have been stored, the signals are integrated (e.g. added) to produce ultrahigh resolution imagery.
Since the integration in SAR and similar radar systems depends on phase coherence of the integrated signals, it is referred at to coherent integration. Typically, ultrahigh resolution SAR requires a long coherent integration time of tens of seconds during which the radar frequency cannot be changed. Because of the long coherent integration time, as well as the wide bandwidth of the transmitted signals, conventional frequency agility and diversity techniques for interference suppression are of limited utility in these systems.
Steering vector adaptive arrays, often called Applebaum arrays, have been utilized to construct radar systems less susceptible to interference. Such arrays generally utilize a feedback loop which adaptively forms antenna pattern nulls in the direction of interference sources. Although such adaptive cancelers are known, much of the effort to date has been directed toward narrowband applications. The narrowband techniques are not directly applicable to wideband radar because increased frequency dispersion causes expansion of the number of adaptive channels as well as problematic exponential increases in computational requirements.